1. Technical Field
This invention relates to the transmission of data over high data-rate point-to-point and point-to-multipoint radio links. Such links are used in many different configurations, using radio transmitter and receivers which may be fixed or mobile, terrestrial or in orbit.
2. Related Art
The major limitations on capacity are available bandwidth and power, coupled with tight constraints on emissions outside the occupied bandwidth. These constraints require a capability to transmit data with high bandwidth efficiency while at the same time being tolerant both to equalisation error (amplitude and group delay) and to non-linearities (such as caused by saturated amplifiers).
There are several methods for increasing the information content of transmissions. These include the use of ever-greater bandwidth, as it becomes technically feasible by making use of higher carrier frequencies. However, carrier frequencies are a scarce resource, and operators are not usually permitted to select a frequency of their choice. Higher frequencies also tend to be subject to greater attenuation and require greater power to generate and amplify.
Some systems, when required to transmit a signal requiring a higher data rate than a single channel can provide, are arranged to make use of two or more channels, each carrying part of the data.
Digital amplitude modulation systems may operate in a binary “on/off” (1/O) mode. To avoid loss of synchronisation with the carrier, it is usual to use two non-zero amplitudes. If these are positive and negative values (of the same or different magnitudes) they may be also considered as a phase shift: a “1” being indicated by a 180° shift in the phase of the carrier signal compared to the phase of a “zero”. Other modulation schemes use other phase shifts, for example using multiples of 45 degrees—known as 8PSK (eight-phase shift keying), as is used in the current state-of-the-art Internet Service Provider (ISP) backbone connection market segment, using Trellis Coded Modulation modems fitting into a 72 MHz satellite transponder to produce a bit rate of 155 Mbit/s. The use of both phase and amplitude shifts is known as Quadrature Amplitude Modulation (QAM).
Such schemes allow several binary digits (“bits”) to be transmitted together, for example a system using a so-called “constellation” of sixteen “symbols”, each reprsented by a different phase/amplitude state, would allow each individual symbol to represent a different four-bit word. The number of modulation states required increases exponentially with the number of binary digits to be represented by each symbol. For ease of conversion, constellations are usually selected to have a number of symbols which is some power of two, so that each symbol can represent a group of binary digits (bits).
International Patent Specification WO02/17507 discloses a system in which data is transmitted in a plurality of channels (data streams), each of which has a multiple-level amplitude modulation system. By selecting the number of levels and the number of channels appropriately, a greater data rate can be achieved for a given bandwidth than using that bandwidth for a larger number of narrowband binary channels, or for a single wide band signal having a larger number of amplitude levels.
Instead of amplitude or phase, different symbols may be represented by different frequencies. This is known as frequency shift keying (FSK). In order to economise on bandwidth, the frequency shifts are generally selected to be the minimum necessary to be distinguishable from each other, (minimum shift keying or MSK). To avoid high frequency spectral spread, the frequency shifts are generally shaped such that the frequency shifts follow a smooth Gaussian waveform rather than a square wave shape: thus the system is known as Gaussian minimum shift keying (GMSK).
In any digital system the individual symbols used must be sufficiently different from each other to be distinguishable by the receiver. However, the greater such a difference is, the more bandwidth is required. It will be apparent that any system other than a simple binary system (or the special case of a 3PSK system) will require that some symbol transitions will be greater than others. The increase in speed obtained by a multiple-level system therefore requires either a greater carrier-to-noise ratio than a simple binary system in order to allow the different levels to be clearly distinguishable, or a greater bandwidth in order to accommodate the same minimum shift between phase/amplitude states. Efficiency of use of bandwidth is obtained only if the gain in speed is greater than the increase in bandwidth required to achieve it. In the case of frequency shift keying a multiple-level system would require an increase in bandwidth proportional to the number of symbols used, if the minimum difference between frequencies is to be maintained, so no advantage is gained compared with the use of a plurality of binary channels occupying the same bandwidth.
However, frequency shift keying has advantages in certain circumstances such as in satellite links, as the cost of setting up earth stations is affected by the required equalisation error across the signal bandwidth and this becomes more of an issue for wide-band signals and in particular for systems using phase shift keying. High tolerance to non-linearities in the channel is also desirable, as satellite channels include one or more elements in series that could be running close to or at saturation point. It is therefore desirable in such circumstances to optimise the bandwidth utilisation of FSK systems
A prior art receiver described by Horikoshi et al in “Error Performance of Improvement of Bandlimited QTFSK Assisted by an Efficient Coded Modulation and Viterbi Sequence Estimation”—IEEE: Proceedings of the Vehicular Technology Conference, Stockholm, Jun. 8-10 1994 (New York) vol 2, conf 44, pages 1006-1010. In the receiver, discrete modulation states are detected by minimising the phase variance at a single decision point (by maximising a so-called “eye opening”), after equalisation has been applied to optimise the symbol shape.